A typical apparatus having a pair of rolls is a calendering device. In such a device, the ends of, usually, one of the rolls are mounted in bearing blocks. The bearing blocks are engaged by spindles. The spindles have external screw-threads which are engaged by a driven spur wheel. By suitably actuating the spur wheel, the roll nip size can be set.
When such a calender is in use, it is of vital importance that the rolls, which are metallic, do not come into contact with one another. This is because such contact damages the polished roll surfaces and necessitates their replacement. The roll pressures produced in a calender may reach values of between 30 and 150 t per bearing journal. If the rolls contact one another, it is almost inevitable that considerable damage may be sustained by the rolls themselves and by the roll bearings.
A calender for plastics material is disclosed in German Patent Specification No. 2 123 507. Such calender includes a device for preventing the rolls from coming into contact with one another. In such calender, control rods are disposed between the rolls, which rods are entrained by the rolls if and when the roll nip size decreases below a predetermined value. When the control rods are entrained by the rolls, a suitable signal is produced which halts the calendering process.
Such a device is not, however, entirely satisfactory either in terms of its speed of operation or its efficiency in preventing damage to the rolls. This is because it is difficult to ensure that the control rods are entrained by the rolls as they move towards one another. Moreover, annular attachments or shoulder portions need to be provided on the roll journals.